A Numerical Study of CO2 Decoupled Effects on Extinction Limit and Flame Microstructure in CH4/Air Counterflow Diffusion Flame with Various Pressures

Extinction limits and flame microstructures of CH4 versus air (N-2/O-2) counterflow diffusion flames with additional CO2 to oxidizer stream under various pressure conditions were numerically investigated by a proposed decoupled strategy. The chemical and thermal effects of CO2 addition were isolated from the synergistic effect on extinction limits and flame microstructures. The results show that the extinction limits were decreased due to the CO2 addition and extended caused by the enhanced pressure, respectively. The changes of flame microstructures from far away from extinction to near extinction mainly are reflected in a narrower combustion zone and thinner flame thickness, while there is no visible displacement of the flame front location. Quantitative analyses manifest the thermal effect of CO2 addition on extinction limit is greater than chemical effect, and elevated pressure can enhance both thermal and chemical effects. An interesting phenomenon occurred between the thermal and chemical effects resulting from increasing CO2 mole fraction in an oxidizer on extinction limit; the former monotonically increase along with CO2 mole fraction, while the latter represents a kind of approximate quadratic-parabola tend along with CO2 mole fraction. The distributions of the production and consumption rate of hydroxyl (OH) radicals were presented to clarify the contribution of the kinetic term in the asymptotic solution to the extinction limits with increasing CO2 molar fraction and various pressures.